Automotive body panels and other sheet metal parts of complex shape can be formed from aluminum alloys of superplastically or quick plastically formable composition and metallurgical microstructure. Superplastic deformation of, for example, Aluminum Alloy 5083 occurs generally between 900 F and 950 F, and the mechanism is grain boundary sliding of very fine grains. Quick plastic deformation of suitable aluminum alloys is described in U.S. Pat. No. 6,253,588, entitled “Quick Plastic Forming of Aluminum Alloy Sheet Metal” to Rashid, et al. Quick plastic forming is practiced at lower temperatures (e.g., 825 F to 875 F) than superplastic forming and often at higher strain rates. In quick plastic forming the deformation is not entirely by grain boundary sliding, it occurs both by grain boundary sliding and dislocation movement. Quick plastic forming produces complex parts with better dimensional quality and reproducibility of the shaped metal than the same parts made by superplastic forming.
Automobile designers and manufacturing engineers cooperate to specify the shape of aluminum alloy body panels that can be successfully formed from the sheet metal. An example of an automotive body panel is a deck lid. A typical deck lid has a generally horizontal surface for covering the top of the vehicle trunk and a generally vertical surface for defining the end of the trunk. Both surfaces usually have a curved shape as they span the vehicle trunk between the opposing vehicle fenders. Furthermore, the deck lid may have a deep pocket shaped recess in the vertical surface for a license plate and for lights that illuminate the plate. Also the deck lid may have a recess at the top of the vertical surface for a center high mounted stop lamp (CHMSL). When a body panel contains such structural features in a single piece of sheet metal consideration must be given to how the metal is formed without wrinkles and tears.
In evaluating the complex shape of such a body panel a finite element analysis can be made of the stretching of the flat sheet metal into the final product. Given the elongation properties of the sheet metal an assessment is made as to whether the part can be made from the available metal stock without tearing or wrinkling of the metal. It is an object of this invention to provide a markedly improved method of using superplastic forming or quick plastic forming as disclosed in the '588 patent to successfully form a sheet metal part of complex shape with a high quality surface.